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Discover how to resolve the `Failed assertion: 'colorScheme?.brightness == null || brightness == null || colorScheme!.brightness == brightness': is not true` error in Flutter by adjusting your dark theme setup.
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This video is based on the question https://stackoverflow.com/q/71069904/ asked by the user 'u2tall' ( https://stackoverflow.com/u/1825233/ ) and on the answer https://stackoverflow.com/a/71070501/ provided by the user 'BJW' ( https://stackoverflow.com/u/9424323/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the Brightness.dark Assertion Error in Flutter Themes

Are you facing an issue with setting the brightness for your Flutter app’s dark theme? Recently, many developers have encountered the assertion error related to the brightness parameter in Flutter's ThemeData. Understanding the cause of this problem and how to resolve it is crucial for ensuring your app runs smoothly and looks great in dark mode.

Understanding the Problem

The error message you're likely seeing is:

[[See Video to Reveal this Text or Code Snippet]]

This error indicates a conflict between the brightness specified in the ThemeData and the brightness defined in your ColorScheme. When using the brightness: Brightness.dark parameter, your ColorScheme must also reflect that it's set to dark, otherwise, Flutter will throw an assertion error.

The Recent Update Impact

This issue arises from a tightening of the ThemeData constructor regarding the brightness parameter and the ColorScheme's brightness parameter in a recent update of Flutter. Before this change, you might have set the brightness directly in ThemeData without issues. However, now both brightness settings must match to avoid the assertion error.

How to Fix the Issue

Step-by-Step Solution

To resolve this assertion error, follow these steps to correctly set your dark theme:

Remove the Brightness Parameter from ThemeData: Instead of specifying the brightness in ThemeData, you will move that into the ColorScheme.

Set Brightness in ColorScheme: Ensure that you correctly set the brightness in your ColorScheme while removing it from the ThemeData declaration.

Here’s how your dark theme should look after making the necessary changes:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Explained

Removal of Brightness from ThemeData: By removing brightness: Brightness.dark from ThemeData, you allow ColorScheme to dictate the brightness state.

Correctly Setting Brightness in ColorScheme: Now, ColorScheme.fromSwatch() includes the brightness property set to Brightness.dark, ensuring consistency between the two properties.

Conclusion

By adjusting your dark theme setup as outlined above, your Flutter application should no longer run into the brightness assertion error. It's crucial to ensure that attributes in ThemeData and ColorScheme are aligned after updates to maintain proper functionality and avoid unexpected errors.

Now that you're equipped with this solution, you can confidently implement dark mode in your Flutter app without running into glitches. Happy coding!